High voltage current limiting fuse



Dec. 7, 1965 LE ROY H. FRANKLIN ETAL 3,222,479

HIGH VOLTAGE CURRENT LIMITING FUSE Filed Jan. 5. 1961 ETA 1/ INVENTORS. 1.: par MFPANKL/N RICHARD M. TRAINER PATENTAGENT United States Patent 3,222,479 HIGH VOLTAGE CURRENT LIMITING FUSE Le Roy H. Franklin, Los Altos, and Richard M. Trainer, Palo Alto, Calif., assignors to Overseas Finance and Trading Company, a corporation of California Filed Jan. 5, 1961, Ser. No. 80,865 6 Claims. (Cl. 200-120) The present invention relates to electrical circuit interrupters, and more particularly, to current limiting fuses for high voltage circuits, that is, circuits operating in the multi-kilovolt range.

If a short circuit occurs in unprotected high voltage electrical equipment, extremely high current (e.g. 50,000 amperes peak) limited only by the existing circuit impedance, can rapidly develop with obvious destructive consequences for many components of the circuit. Accordingly, current limiting fuses are inserted in such high voltage circuits in an attempt to provide, upon the occurrence of short circuit or major fault currents, a very high resistance to limit such current to a lower, non-destructive value.

Presently available fuses fail to achieve the desired current limiting function quite frequently in at least one of the two following ways. Initially, they have failed, upon the occurrence of a short circuit, to-blow with sufiicient rapidity to insert the required high resistance in the circuit in sufiicient time to preclude the rise of the fault current to a destructive value. Secondly, such fuses, after they have iblown, fail to clear properly so that an arc is restruck across the fuse terminals with consequent destructive effect, Such restriking of an arc occurs more readily in direct current equipment since the peak current is maintained; wherefore it will be obvious that successful fuse operation in direct current circuits is exceedingly difiicult to attain.

The commercially-available fuses subject to the aforementioned failures, particularly in direct current circuits, generally include the following two essential elements; a fusible element normally in the form of a length of wire composed of silver or other highly conductive metal and a surrounding body of chemically inert filler material, generally quartz sand. While the very inconsistent results obtainable in the operation of these commercially-available fuses renders a technically accurate explication of their functioning exceedingly difficult, it is believed that the noted failures arise somewhat in the following fashion. When the short circuit occurs, the cur-rent increases rapidly in value to effect first fusion and then volatilization of the silver wire or other fusible element and thereafter forms a very high temperature arc. The concomitant rise in pressure disperses the volatilized silver into the adjacent body of quartz sand whose temperature is in turn raised considerably to produce a fulgurite or fusion shell whose inherent resistance is considerably below that of quartz sand at normal ambient temperatures. As a consequence, if continuity of such fusion shell exists between the two terminals of the fuse, the fault current is not limited to the desired value. Furthermore, although the quartz sand is presumably an arc-quenching material and may indeed quench the initial arc, the creation of the noted fusion shell may permit the restriking of a secondary arc between the fuse terminals.

Whether the foregoing explanation of the operational failure of commercially-available fuses be correct or not, it is known that such failures do occur and accordingly, it is the general object of the present invention to provide a high-voltage, current limiting fuse which, when exposed to short circuit or other major fault currents, will blow rapidly to insert an extremely high resistance into the fused circuit to limit the current to non-destructive values, and furthermore, will clear completely so that such high resistance value will be maintained and thus continue to limit the current to such non-destructive values.

More particularly, it is a feature of the invention to provide a high voltage, current limiting fuse including a conductive fusible element and an encompassing body of pulverulent filler material physically associated in a manner such that upon blowing of the fuse the volatilized fusible element will be dispersed in a discontinuous manner throughout the filler material.

Additionally, it is a feature of the invention to provide a high-voltage, current limiting fuse wherein the mentioned fusible element, during the blowing and clearing operation of the fuse, undergoes, a chemical reaction, the product or products of which constitute non-conductors. The term non-conductor as employed in this specification is intended to refer to a material whose resistive value is sufficiently high to produce the required current limiting action.

Additionally, it is a feature of the invention to provide a high voltage, current limiting fuse wherein substantially no moisture exists within the fuse prior to the blowing thereof, and furthermore, is not created as one product of the aforementioned chemical reaction during the blowing and clearing of the fuse.

A related feature of the invention provides for the sealed construction of the fuse rendering the same impervious to the entry of moisture from the surrounding environment and also provides for the incorporation of a predetermined amount of a desiccant within the sealed interior of the fuse to capture any free moisture existing therewithin.

An additional and very practical feature of the present invention is the provision of a high voltage, current limiting fuse whose physical arrangement enables the minimization of overall fuse dimensions requisite for a given current and voltage rating.

Yet another feature of the invention relates to the provision of a high voltage current limiting fuse whose design enables ready adaptability to a wide range of voltage and current requirements up to three hundred kilovolts and 35 amperes, or more.

These as well as other objects and features of the invention will become more apparent from a perusal of the following description of the structures illustrated in the accompanying drawing wherein:

FIG. 1 is a side elevational view of a fuse structure em- =bodying the present invention, portions thereof being broken away and shown in section to illustrate interior details of construction,

FIG. 2 is a transverse sectional view taken along line 2-2 of FIG. 1,

FIG. 3 is a side elevational view of a fuse constituting a modified embodiment of the invention, portions of the fuse again being broken away and illustrated in section for clarification of interior details, and

FIG. 4 is a transverse sectional view taken along line 44 of FIG. 3.

Generally, a high voltage, current limiting fuse embodying the present invention includes a fusible element substantially entirely encompassed within a pulvenllent filler material such that upon blowing and clearing of the fuse resulting from the occurrence of a major fault current in the circuit of which the fuse forms a part, the following two conditions are preferably satisfied: (l) a discontinuous dispersion of the fusible element within the filler material is experienced, and (2) a chemical reaction occurs between the fusible element and the filler material, the reaction products of which are non-conductors, wherefore the ultimate objective of inserting a very high resistance in the circuit is met.

More particularly, there is illustrated in FIGS. 1 and 2 in approximately full scale representation, a fuse having a rating of 25 kilovolts and two amperes R.M.S. and constituting one embodiment of the present invention. As illustrated, the fuse is of generally cylindrical configuration including a tubular housing composed of an impervious insulator material such as a phenolic plastic, such housing'10 being sealingly closed at its opposite ends by cup-shaped metallic end caps 12, 14 of copper or other material which serve to form the terminals of the fuse.

Within the tubular housing 10, a pair of silver wires 16 and 18 jointly form the fusible element of the unit and are each connected at their respective ends to the described terminal caps 12, 14. More particularly, as illustrated, each of the terminal caps 12, 14 is centrally depressed and provided with a small opening through which the ends of the silver wires 16, 18 can be passed whereupon solder can be applied to complete the connections, as indicated at 12a and 14a. Between the terminal connections, each of the silver wires 16, 18 pass through the housing 10 in parallel relationship along a sinuous, zig-zag path, the wires being supported in such configuration by a multiplicity of stacked and interdigitated disks 20, composed of a heat-resistant insulator material, such as ceramic.

As shown best in FIG. 2, each of the disks 20 includes two parallel edges joining a third edge of arcuate configuration conforming closely to the interior curved surface of the tubular housing 10 and a fourth edge spaced inwardly from the housing and containing a plurality of notches, as indicated at 22 in FIG. 2. Adjacent the four corners of the disk, integral protruding legs or spacer elements 24 are formed which serve upon stacking of the disks 20 to maintain spacing between the main body portions of adjacent disks, as best illustrated in FIG. 1. In order to obtain the mentioned interdigitated arrangement of the disks 20, adjacent disks 20 are disposed oppositely within the housing 10, thus allowing the silver wires 16, 18 to be arranged in the described zig-zag path, engaging the notches 22 at the one edge of each disk and then extending between the spaced disks to the notches 22 at the oppositely disposed edge of the underlying disk and so on until the complete zig-zag transit is completed between the upper and lower fuse terminals 12, 14. As will be obvious, the notches 22 serve to maintain the required parallel spacing of the two silver wires 16, 18, as clearly illustrated in FIG. 2. For reasons to become apparent hereinafter, it should be observed particularly that the wires 16, 18 contact the ceramic disks 20 only at that position where the notches 22 at the one edge of each disk are engaged.

The entire space within the tubular housing 10 and between the terminal caps 12, 14 that is not occupied by the disks 20 and the wires 16, 18 is packed with a pulverulent filler material, indicated at 26. The term, pulverulent, as used herein refers to a plurality of particles which vary from a very fine powder to a more or less granular size corresponding to that of normal table salt, the prefered size of the particles depending upon the precise chemical nature of the material. In accordance with previously known principles, the filler material 26 should have high dielectric strength, good thermal conductivity, and high heat capacity. Additionally, in accordance with the present invention, the material 26 should be a non-conductor before blowing of the fuse and should be capable of chemically reacting rapidly with the volatilized silver during the blowing of the fuse to create a reaction product or products which also are nonconductive. Elemental sulphur is one excellent material; at the extremely high temperatures attained during blowing of the fuse, an extremely rapid chemical reaction occurs to produce the non-conductive silver sulphide. Many obvious alternatives for use as the filler material will occur to the chemist; sodium chloride can be used, in which case, the chemical reaction produces silver chloride; ferric oxide can also be utilized, in which case, the

chemical reaction is such as to produce a combination of silver oxide and ferrous oxide, both of which are nonconductors in the sense as defined hereinabove.

While silver is an excellent material for the fusible element of a high voltage, current limiting fuse, it is conceivable that in some instances aluminum, or some other metal, might be alternatively employed, and the element or compound chosen for the filler material must be such as to be a non-conductor before blowing of the fuse and again during blowing of the fuse provide reaction products which are non-conductive.

As has been mentioned, sulphur is an excellent filler material for use with the described silver wires 16, 18, but it is known that some reaction of the silver with the sulphur occurs even at room temperatures, Accordingly, a thin fusible coating can, if desired, be applied over the silver wires 16, 18 to preclude the occurrence of such reaction under normal conditions. For this purpose, the silver wire could be dipped and provided with a thin coating of Teflon, a non-conductive plastic material which will readily fuse and disperse during blowing of the fuse so that the chemical reaction between the silver and the sulphur can proceed in the manner described. Alternatively, a conductive material such as gold can be applied, as by plating, over the silver wires 16, 18. At room temperature, the sulphur and silver are separated but upon blowing of the fuse, the gold will volatilize with the silver and permit the silver and sulphur reaction to proceed in the manner described. During the blowing of the fuse, the small amount of gold will be volatilized and be dispersed in a discontinuous fashion between the fuse terminals, as will become more apparent hereinafter, so that the required high resistance between the fuse terminals will be obtained.

The sulphur or other material should be thoroughly dried before assembly of the described fuse and such assembly should preferably take place in a dry atmosphere so that upon subsequent sealing of the terminal caps 12, 14 over the tubular housing 10, a moisture content below one-tenth of one percent by weight is achieved. To maintain the moisture at this low level, a desiccant, such as activated barium oxide, can be mixed with the sulphur or other filler material in an amount approximating onehalf percent by weight.

While under most conditions, the interstices between the particles of the filler material 26 can be filled with dry air without any deleterious consequences, such interstices can, if desired, be filled with an electronegative gas, such as sulphur hexafluoride, during the assembly operation, such gas serving to capture any free electrons generated during blowing of the fuse and to produce heavy ions of low mobility in an electrically-known fashion.

When the fuse illustrated in FIGS. 1 and 2 has been appropriately inserted into a piece of electronic equipment and a short circuit or major fault current arises, the fuse blows and clears in substantially the following fashion to rapidly insert a very high resistance into the circuit to preclude damage to its components. The fault current rises rapidly to produce first fusion and subsequently volatilization of the silver wires 16, 18. Parallel zig-zag arc paths will be created along the zigzag paths previously occupied by the two parallel wires, the disposition of the ceramic disks 20 obviously restricting the arcs to such lengthy paths. Since two wires 16, 18 are utilized, their diameters can be smaller for the required voltage and current requirements; wherefore the described volatilization, arc production, and consequent dispersion will occur with greater rapidity. It may be mentioned that the spacing between the parallel wires 16, 18 is sufficiently great so that the interposed filler material 26 prohibits any interaction effect and thus each wire can be considered as acting independently during the blowing and clearing function.

When the arc is generated, a rapid temperature rise is experienced with a concomitant rapid expansion of the volatilized silver so that the latter is forcibly dispersed into the surrounding pulverulent filler material 26. The intense heat plus the intimate contact between the volatilized silver and the sulphur produces an extremely rapid reaction, the product of which is the non-conductive silver sulphide, as has been previously mentioned. The dispersement of the volatilized silver into the sulphur is complete except at those spaced points where the silver wires 16, 18 contact the notches 22 at the edges of the ceramic disks 20. It is then conceivable that a small amount of silver in its metallic form might condense at the edges of the inert ceramic material. However, between this silver deposited at the notched edge of one disk and at the edge of a similarly disposed disk, an intermediate disk is disposed in insulating relationship. Thus even though some free silver be deposited at the notched edges of the disks 20, a relatively long insulated path exists between such metal deposits so that the subsequent reformation or restriking of a secondary arc is effectively precluded.

Thus, in the blowing and clearing of the described high voltage current limiting fuse, not only is most of the silver combined into a non-conductive compound, but what free silver does remain is spatially separated a considerable distance. As a consequence, the specifically described fuse is capable of blowing and clearing in a period as short as one microsecond, such period being, of course, partially dependent upon the total impedance of the exterior circuit.

The described structure readily adapts itself for other current and voltage ratings. If the voltage rating is the same but lesser current is permitted, a single silver wire can be arranged between the terminal caps whereas if a higher current rating is required, perhaps, for example, four wires can be arranged in parallelism, still utilizing the same basic structure, as illustrated in FIGS. 1 and 2.

On the other hand, if a higher voltage requirement, for example, 75 kilovolts, is to be met but with the same current rating as previously mentioned, three of the units, as illustrated in FIGS. 1 and 2, can be stacked in series. As an obvious alternative, a longer single fuse can be employed for the 75 kilovolt requirement.

Additionally, it will be apparent that the principles embodied in the structure illustrated in FIGS. 1 and 2 can be embodied in alternate physical forms. One such alternative is illustrated in FIGS. 3 and 4 where corresponding elements are indicated by like numerals. The tubular housing and the end caps 12 and 14 are identical with those described with respect to the first embodiment of the invention and the ends of the silver wires 16 and 18 are similarly secured to the terminal caps, as indicated at 12a and 14a.

However, in contradistinction to the zigzag formation of the wires as supported by the disks 20 in the FIGS. 1 and 2 structure, the wires 16 and 18 are wound in a helical configuration about a single mandril that extends the full length of the housing 10 between the terminal caps 12, 14, being positioned substantially coaxially therein. This mandril 30 is preferably composed of ceramic material and is provided with a multiplicity of parallel, longitudinal slots 32 along its periphery so that in cross-section, as shown in FIG. 4, the mandril is of star-shaped configuration. Thus, the helically-wound wires 16, 18 contact the mandril 30 only at a plurality of spaced points, and between these points, the wire are completely encompassed by the filter material 26, which, as in the first embodiment of the invention, is chemically reactive so as to be capable, upon blowing of the fuse, of producing non-conductive reaction products.

Thus, in the blowing of this fuse, most of the silver is transformed into a non-conductive compound, but that which may remain in the form of free silver on the supporting mandril 30 is spatially separated from any other free silver by non-conductive material so that the restrik- 6 ing of an are after blowing of a fuse is effectively precluded.

Obviously many further modifications or alterations can be made in the described structure without departing from the spirit of the invention, and the foregoing description of two embodiments is to be considered as purely exemplary and not in a limiting sense. The actual scope of the invention is to be indicated by reference to the appended claims.

What is claimed is:

'1. A high voltage fuse which comprises an impervious housing having two spaced fuse terminals, a fusible element within said housing and having its opposite extremities connected to said terminals, said fusible element being composed of a conductive material capable of v0latili zation under fuse-blowing conditions, a body of pulverulent filter material filling said housing and substantially entirely encompassing said fusible element, said filler material being a non-conductor selected from the group consisting of sulphur, sodium chloride and ferric oxide and capable of chemically reacting rapidly with said fusible element when volatilized under fuse-blowing conditions, the product of the chemical reaction being non-conductive, said filler material having a negligible moisture content.

2. A high voltage fuse according to claim 1 which comprises a desiccant mixed with said filler material.

3. A high voltage fuse which comprises an impervious housing having two spaced fuse terminal-s, a fusible element within said housing and having its opposite extremities connected to said terminals, said fusible element being composed of a conductive material capable of volatilization under fuse-blowing conditions, a body of pulverulent filler material filling said housing and substantially entirely encompassing said fusible element, said filler material being a non-conductor and capable of chemically reacting rapidly with said fusible element when volatilized under fuse-blowing conditions, the product of the chemical reaction being non-conductive, and an electro-negative gas filling the interstices between the particles of said pulverulent filler material.

4. A high voltage fuse which comprises an impervious housing, a fusible element in said housing and composed of a conductive material capable of volatilization under fuse-blowing conditions, a body of pulverulent filler material substantially entirely filling said housing and closely encompasisng said fusible element, said filler material being a non-conductor selected from the group consisting of sulphur, sodium chloride and ferric oxide and capable of chemically reacting rapidly with said fusible element when volatilized under fuse-blowing conditions, the product of the chemical reaction being nonconductive, said impervious housing entirely enclosing said fusible element and said filler material.

5. A high voltage fuse according to claim 4 which comprises a thin coating of fusible material on the surface of said fusible element, said coating material being chemically non-reactive with said filler material at standard temperatures.

6. A high voltage fuse which comprises an insulator housing, a pair of conductive fuse terminals at opposite ends of said housing, a fusible element composed of a conductive material capable of volatilizatoin under fuseblowing conditions extending through said housing and connected at its opposite ends to said terminals, and means including an insulator engaging and supporting said fusible element at a plurality of spaced points intermediate its terminal connections, and a body of pulverulent filler material substantially entirely encompassing said fusible element between the points of support, said filler material being a non-conductor and capable of chemically reacting rapidly with said fusible element when volatilized under fuse-blowing conditions, the product of said chemical reaction being non-conductive, said supporting insulator consisting of a series of interdigitated spaced disks arranged to support said fusible element in 7 a zig-zag configuration, the spacing between disks being just large enough to accommodate said fusible element and each disk having substantially an equivalent thickness to the spacing therebetween.

References Cited by the Examiner UNITED STATES PATENTS 737,284 8/1903 Sachs 200-131 1,819,198 8/1931 Reichmann et a1. 200-120 2,018,556 10/1935 Hope 200-131 2,143,031 1/1939 Rapp 200-120 2,285,602 6/1942 McMahon 200-120 2,294,132 8/1942 Schuck 200-120 8 Miller 200-120 Edsall et a1. 200-120 Fahnoe et a1. 200-120 Fister 200-131 Jacobs et a1. 200-131 Jacobs 200-131 FOREIGN PATENTS France.

BERNARD A. GILHEANY, Primary Examiner.

MAX L. LEVY, Examiner. 

1. A HIGH VOLTAGE FUSE WHICH COMPRISES AN IMPERVIOUS HOUSING HAVING TWO SPACED FUSE TERMINALS, A FUSIBLE ELEMENT WITHIN SAID HOUSING AND HAVING ITS OPPOSITE EXTREMITIES CONNECTED TO SAID TERMINALS, SAIKD FUSIBLE ELEMENT BEING COMPOSED OF A CONDUCTIVE MATERIAL CAPABLE OF VOLATILIZATION UNDER FUSE-BLOWING CONDITIONS, A BODY OF PULVERULENT FILTER MATERIAL FILLING SAID HOUSING AND SUBSTANTIALLY ENTIRELY ENCOMPASSING SAID FUSIBLE ELEMENT, SAID FILLER MATERIAL BEING A NON-CONDUCTOR SELECTED FROM THE GROUP CONSISTING OF SULPHUR, SODIUM CHLORIDE AND FERRIC OXIDE AND CAPABLE OF CHEMICALLY REACTING RAPIDLY WITH SAID FUSIBLE ELEMENT WHEN VOLATILIZED UNDER FUSE-BLOWING CONDITIONS, THE PRODUCT OF THE CHEMICAL REACTION BEING NON-CONDUCTIVE, SAID FILLER MATERIAL HAVING A NEGLIGIBLE MOISTURE CONTENT. 